(1) Field of the Invention
The present invention relates to a semiconductor device, more particularly to a semiconductor device which is used, for example, for controlling and driving thermal heads of a thermal printer used in a facsimile apparatus and which has power supplied to part of the internal circuits only when necessary.
(2) Description of the Prior Art
In general, a semiconductor device comprises a plurality of circuit portions. Some of the circuit portions require a continuous supply of power, while other circuit portions require a supply of power only during part of the operating time of the semiconductor device. When a semiconductor device is provided with a single switch circuit which turns the power on and off for all the circuit portions together, there is no problem with the above-mentioned former circuit portions, but power is wasted on the latter circuit portions. The result is a larger than necessary power consumption and heat generation in the semiconductor device.
Semiconductors are used, for example, in thermal printers used in facsimile apparatuses. Specifically, integrated circuit (IC) devices are used to supply signals to thermal heads to control the drive thereof. After the printing of characters in a line is finished, the printing paper is moved a predetermined length and the printing of a next line is effected.
In a conventional facsimile apparatus, a power supply voltage is applied to the IC devices even when not necessary, i.e., even when the printing paper is being moved and the printing operation by the thermal heads is not effected. The time for movement of the printing paper is about seven times as long as the time for the printing operation by the thermal heads. Therefore, the conventional IC device for controlling and driving the thermal heads wastes power and generates much heat.